The present invention relates generally to an internal combustion engine and, more particularly, to a turbo-compound internal combustion engine.
For turbo-superchargers commonly used with internal combustion engines, the flow rate of exhaust air at low engine speed generally is insufficient resulting in less than satisfactory performance. More specifically, the ratio P.sub.2 /P.sub.1 between inlet pressure and outlet pressure with respect to the flow rate of exhaust air to the turbo-supercharger varies as shown in FIG. 3. Since the efficiency of a supercharger's turbine is expressed by n=1-(P.sub.1 /P.sub.2) .sup.K-1/K, the ratio of inlet pressure and outlet pressure substantially affects efficiency with increased inlet pressure providing increased turbine efficiency. Accordingly, for internal combustion engines exhibiting large variations in normal exhaust gas flow rates, it is very difficult to effectively utilize exhaust energy for intake supercharging when using a single turbine.
Laid-Open Japanese Utility Model application No. 114844/1983 discloses an arrangement wherein a turbo-supercharger turbine disc possesses both a low-speed disc and a high-speed disc disposed internally and externally, respectively. A partitioning wall in the turbine housing provides separate passages to each of the high-speed and low-speed turbine discs and a valve interrupts flow to the high-speed disc when the flow rate of exhaust gases is small. An improvement of that device is disclosed in U.S. patent application Ser. No. 920,456, entitled "Turbo-Supercharger for Internal Combustion Engine" filed concurrently with this application, and having a common assignee. Although the above noted structural arrangements improve turbo-supercharger efficiency, they are not readily applicable to a turbo-compound internal combustion engine.
In the turbo-compound internal combustion engine wherein exhaust gases from a turbo-supercharger are introduced into a recovery turbine coupled to a crank shaft, the pressure ratio between the turbo-supercharger and the recovery turbine is a particularly important factor. As shown in FIG. 3, with a pressure ratio A in the turbo-supercharger, the pressure ratio B of the recovery turbine also will be maintained sufficiently high. However, a reduced flow rate of exhaust gases produces a lower pressure ratio C in the turbo-supercharger and a still lower pressure ratio D in the recovery turbine. Thus, in order to increase the efficiency of the recovery turbine, it is important to shift the recovery turbine pressure ratio from the value D to the value E.
The object of the invention, therefore, is to provide a turbo-compound internal combustion engine in which the pressure ratio and the efficiency of the recovery turbine is increased during low rotational speed operation of the engine.